Implantable medical devices (IMD) may provide a variety of different therapies and other functions including stimulation, drug infusion, physiological sensing, and the like. The IMDs receive programming from an external device and may also share information that has been collected with the external device. Many IMD communicates with the external device using an inductive form of telemetry where a telemetry head is held in communication range of the IMD so that inductive signals may be exchanged.
IMDs operate on battery power and therefore have a limited lifetime of operation before a replacement or a recharge is necessary. For IMDs that are capable of recharging the battery, the recharge energy is also received via an inductive coupling. The external device has a coil tuned to a recharge frequency, e.g., 5 kilohertz, that differs from the telemetry frequency. The IMD conventionally has a second coil that is tuned to the recharge frequency being emitted by the external device.
During the recharge process, an excessive amount of power may be coupled into the coil for various reasons. For example, the external device may be providing more power than is needed. As another example, the recharge coil of the IMD may be receiving energy from additional nearby sources of inductive energy. In such a case, there may be an overcharge condition that occurs where there is the potential to supply an excessive current or voltage to the battery.